1. Field of the Invention
This invention relates to a method for developing light-sensitive lithographic printing plates. More particularly it is concerned with a method for removing unexposed areas formed in a light-sensitive lithographic printing plate by imagewise application of actinic radiation thereon using a developer composition.
2. Description of the Prior Art
In a lithographic printing plate, oleophilic image areas materially corresponding to an image to be printed and hydrophilic nonimage areas are formed in the same plane. The oleophilic image areas are ink-receptive and water-repellent, whereas the hydrophilic nonimage areas are water-receptive and greasy ink-repellent. An ink pattern is transferred directly or through a blanket to a paper.
In order to photographically form an image, presensitized (PS) plates and wipe-on plates are widely used as light-sensitive lithographic printing plates for photoengraving. These plate materials can be classified as additive type materials in which an emulsion lacquer is coated to reinforce the image after development and subtractive type materials in which a light-sensitive layer having the desired abrasion resistance is previously provided and nonimage areas are selectively removed at development. Developers of the present invention are mainly used in the latter subtractive type materials. In printing plate materials of the latter subtractive type in which diazo resins are used as photosensitive materials, hydrophobic resins are usually used as binders.
A light-sensitive composition is thinly coated on a support having a hydrophilic surface. When the light-sensitive composition thus coated is imagewise exposed through a transparent negative film, only the exposed areas harden and become solvent-insoluble in contrast with the unexposed areas. This leads to a difference in properties with respect to solvents between the exposed areas and the unexposed areas. By utilizing this difference, an image pattern on the exposed areas alone is selectively formed on the hydrophilic surface of the support whereby a printing plate is obtained.
The hardening of diazo resins coated on the exposed areas, however, is insufficient. Usually, when the diazo resin is immersed in a solvent for use in coating the light-sensitive composition, a large portion of the cured resin is dissolved in the solvent or is severely swollen and removed to such an extent that only the diazo decomposition products on the exposed areas remain on the surface of the support. Such printing plates have no practical use. Therefore, it is desired for a developer for a lithographic printing plate, which is made from a light-sensitive composition comprising a diazo resin and a hydrophobic resin, to thereby increase as much as possible the difference in solubility or degree of swelling between the exposed areas and the unexposed areas. That is to say, desirable developer compositions are those which remove completely the unexposed areas and do not deteriorate at all the printing capability of the exposed areas.
The developer as described above must also be capable of development completion in a short period of time. Recently the need for this characteristic has been increasing as automatic developing machines become more widely used. The reason for this is that since an automatic developing machine is designed so that a large number of plate materials can be processed in a short period of time, the developing conditions are always constant and it is not possible to vary the time of development and the strength of rubbing unlike the case in which development is carried out by flowing a developer on a printing plate and manually rubbing the surface.
Conditions under which a developer is used markedly vary. For example, the developer is used at temperatures of less than about 10.degree. C. in cold areas in the winter. Insufficient development is not acceptable even under these conditions. If there is any residue on the unexposed areas, ink will adhere at the time of printing, scumming the background of the print. Similarly, a lapse of several months from the production of printing plates tends to cause scumming to be formed.
Therefore, development has long been desired with those developers which meet the above requirement and, furthermore, which minimize or eliminate problems in the working environment such as pollution due to waste developer, odor, working sanitation, etc., problems of hazards in transfer, storage, etc., problems of production cost, etc.
An aqueous solution type developer containing an anionic type surface active agent, benzyl alcohol and an alkali agent has been proposed in Japanese Patent Application (OPI) No. 77401/1976. While this developer has satisfied substantially all of the above requirements and achieved commercial success, it has the disadvantage in that where a printing plate material is developed with the developer after a lapse of several months from the production of the printing plate material, unfavorable conditions arise, resulting in the formation of scumming.
U.S. Pat. No. 3,954,472 describes an aqueous developer comprising an alcohol, ammonium sulfite and polyvinyl pyrrolidone for use in negative type subtractive light-sensitive lithographic printing plates. With a printing plate produced using this developer composition, no scumming is formed in the nonimage areas, but its resistance to printing is insufficient as compared with a printing plate developed with a developer composition described, for example, in Japanese Patent Application (OPI) No. 77401/1976. Furthermore, it is considered that since a large amount of water-soluble low boiling point alcohols such as n-propyl alcohol and the like are present in the developer described in a specific example of U.S. Pat. No. 3,954,472, problems of working sanitation, hazards and pollution arise due to odor and the relatively low flash point of n-propyl alcohol and the like.